Bioluminescent-like squamation in the galeomorph shark Apristurus ampliceps (Chondrichthyes: Elasmobranchii)

Galeomorph sharks constitute the most taxonomically and ecologically diverse superorder of living selachians. Despite comprising several typically deep-water taxa, no bioluminescent species have been reported in this group so far. Interestingly, the study of shark squamation has been revealed in recent years to be a good proxy for inferring some ecological aspects of poorly known species. In particular, the high morphological specificity of the dermal denticles and the squamation patterns of all currently-known bioluminescent sharks could constitute a potential tool for predicting bioluminescence in both fossil and living taxa. Following this idea, we provide the first evidence supporting the possible existence of bioluminescence among galeomorph sharks by means of the quantitative study of Apristurus ampliceps squamation pattern. Classical morphometric analysis and multivariate statistical procedures have allowed us to determine that A. ampliceps squamation, composed mainly of bristle-shaped dermal denticles, is highly convergent with that of the bioluminescent shark Etmopterus spinax. The ecology of A. ampliceps, being a species that exclusively inhabits aphotic waters, is in agreement with such a morphofunctional interpretation, but finding photophores is imperative to confirm this prediction.